Overheating and coolant not circulating?

sometimes when my car sit for hours like over night and when I first start it up. there is a high squealing noise for like 5 seconds or so. when I drive my car for 2hours or more my car starts to overheat especially if I'm waiting at a stoplight. I pull over and waited for the car to cool down. I opened my... show more sometimes when my car sit for hours like over night and when I first start it up. there is a high squealing noise for like 5 seconds or so. when I drive my car for 2hours or more my car starts to overheat especially if I'm waiting at a stoplight. I pull over and waited for the car to cool down. I opened my hood and look in the coolant resirvore and it was full max out all the way to the top. then I opened the radiator cap and look in I cant see any coolant. I have to add coolant to the radiator. It usually take like half a quart of coolant to fill it back up. I go back to start my car and the temperature went back to normal. like I said this only happens if I drive for 2 hours or more straight then the temp gauge will start to go up towards the H. what is the cause of this overheatting?,
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